Karolina Muchová won the Bad Homburg grass court tournament on Saturday when opponent Naomi Osaka retired injured early in the second set.
Former French Open finalist Muchová was leading 6-1, 1-0 in the first grass court final for both players when Osaka could not play on after 46 minutes due to a foot injury, giving Muchová a third career title.
Osaka took a medical time-out after the third game to have treatment on her right foot. But the former world number one lasted only five more games, possibly also not wanting to take any risks two days before the start of the Wimbledon grand slam.
"I apologise for not being able to finish," Osaka said, adding: "Congratulations Karolina, you are one of the coolest girls on the tour. Congratulations for an incredible tournament."
The start of the final was put forward by two-and-a-half hours to 11am (0900 GMT) due to the heatwave engulfing many parts of Germany.
